In any network in which individual devices operate using their own clock, signals from each of the devices can have different operating phases. Because of small variations in clock frequencies of each device, variations in start phases for the clocks in each device, and a variable propagation distance for any given signal, an incoming signal will have an indeterminate phase from the point of view of the receiving device. It is therefore necessary for any receiver device to first identify the phase of an incoming signal before it can be properly processed.
One way to achieve this phase identification is to have the transmitting device begin a transmission by sending a known data pattern (e.g., a preamble) that has a good autocorrelation property. The receiving device can then create a local copy of the known sequence at a known phase and correlate it with the incoming signal. The receiving device can then step through some or all of the full three hundred and sixty degrees of possible phase for the local signal (in known phase steps), calculating correlation functions between the incoming signal and the local signal for each separate local signal phase. A point with a sufficiently high correlation value will indicate that the local signal is sufficiently close in phase to that of the incoming signal to allow proper processing.
In a multipath environment there may be multiple copies of the same incoming signal, each potentially at a slightly different phase. In this case there will be multiple “acceptable” local phases corresponding to multiple strong copies of the incoming signal. The receiving device must then choose one of these as an operating phase for its local clock when processing the associated incoming signal.
Alternate embodiments could use multiple receiving arms in this case, allowing a receiving device to process signals over multiple incoming signal paths. In this case, the receiving device must determine an operating phase for each of the signals being processed by each of the receiving arms.
However, because of the realities of device operation, there will be a delay between when the receiving device determines an acceptable local phase for processing an incoming signal and when it can instruct its local clock to stop varying the local phase. This delay may cause the local clock to settle on a phase that is one or more phase steps removed from the actual best phase. It would be desirable to eliminate this unwanted phase error.